models.js

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const sortAnswers = function(a, b) {
  // - negative a before b
  // 0 no change
  // + positive a after b
  if (a.votes === b.votes) return b.updatedAt - a.updatedAt
  return b.votes - a.votes
}

const AnswerSchema = new Schema({
  text: String,
  createdAt: { type: Date, default: Date.now },
  updatedAt: { type: Date, default: Date.now },
  votes: { type: Number, default: 0 },
})

AnswerSchema.method('update', function(updates, callback) {
  Object.assign(this, updates, { updatedAt: new Date() })
  this.parent().save(callback)
})

AnswerSchema.method('vote', function(vote, callback) {
  if (vote === 'up') {
    this.votes += 1
  } else {
    this.votes -= 1
  }
  this.parent().save(callback)
})

const QuestionSchema = new Schema({
  text: String,
  createdAt: { type: Date, default: Date.now },
  answers: [AnswerSchema],
})

QuestionSchema.pre('save', function(next) {
  this.answers.sort(sortAnswers)
  next()
})

const Question = mongoose.model('Question', QuestionSchema)

module.exports.Question = Question